Articles for use at high temperatures



- severe corrosive conditions.

Patented Apr. 16, 1946 UNITED STATES PATENT OFFICE 2.398.102 narrows For: USE AT HIGH TEMPERA- 'rmuss Martin Fleischmann, Canton, Ohio, asslznor to The Timken Roller Bearing Company, Canton,

Ohio, a corporation of Ohio No Drawing. Application February 26, 1941,

Serial ltl'o. 380,599

8 Claims.

This invention relates to articles operated at elevated temperatures, especially metallic parts which are highly stressed or are exposed to oxidation at high temperatures, and to iron alloys applicant, which are markedly superior in creep strength to the 18-8 steels and are thus better adapted to bear load at high temperatures. Despite the fact that there have been available such steels, their properties have been inadequate for various high temperature uses, more particularly at temperatures above those at which the previously available steels can be operated satisfactorily.

More in detail, there is a demand for alloys which are more resistant to high temperatures than those available prior to this invention. One example of this is to be found in the case of exhaust operated aircraft superchargers, certain parts of which are destroyed quite rapidly under the influence of the high temperatures to which they are exposed. A typical example is to be found also in the case of gas turbines. Up to the time of this invention the maximum temperature at which such devices could be operated 'economically was in the vicinity of 1100 F. Under such conditions of operation the thermal efficiency of such turbines is of the order of about 7 to 9 per cent, for which reason such turbines are at present used only to a restricted extent. However, it is known that if gas turbines could be operated. at temperatures about 300 F. higher their efliciency would be increased to at least about 30 per cent, and in such case they could compete successfully with, and even replace in many instances, steam turbines, with consequent great advantage, particularly where space or weight factors are involved. Thus the steam generating equipment of naval vessels could be eliminated, which would be highly desirable. For such operation, however, the first few stages of the turbines would operate at temperatures approaching 1400 F., while the turbine shaft ability at such temperatures or because 01. severe scaling with consequent rapid destruction.

It is among the objects of the invention to provide articles for use at high temperatures, which possess a combination of desirable mechanical properties at room temperature and excellent creep strength, adequate resistance to oxidation, and high rupture strength, and which are adapted for satisfactory operation at temperatures and under stresses higher'than are possible with previously known steels.

Another object is to provide austenitic steels of relatively simple. composition, which are capable of being wrought to shape, which possess good mechanical properties at room temperature and higher creep and rupture strengths than have been available heretofore in austenitic steels, and which in use at elevated temperatures are capable of bearing greater loads at a given temperature and of operating at higher temperatures than the steels known prior to this invention.

Other objects will appear from the following description.

The invention is predicated upon my discovery that its objects are attained by forming high temperature articles from iron alloys containing carbon in the amounts used in steels together with nickel, chromium and molybdenum in certain amounts within relatively narrow ranges and so balanced with respect to one another that the alloys areaustenitic and free, or effectively free, from delta iron. I have found, and as will appear it has been shown by actual test, that alloys made in accordance with this invention are forgeable, possess desirable mechanical properties at room temperatures, and are not only resistant to scaling at high temperatures but also are decidedly superior in load bearing capacity 4 poses, as where high scale resistance is not essential, the content of chromium maybe lowered somewhat, say to 12 per cent, although for most purposes it is preferred that this element be presentwithin the range stated. The content of D nickel will depend upon the amounts of chromium, molybdenum, carbon, manganese and silicon present in the alloys, but in accordance with the invention it is present in an amount such as to prevent the development of ferrite and delta iron due to the high content of molybdenum inthe presence of the large amount of chromium contained in the alloys, so that the alloy will be austenitic. The amount of nickel necessary will increase, generally speaking, with increase in the so content of chromium and molybdenum. When for special purposes, as indicated above, the

chromium content is as little as 12 per cent, the

'range stated. The use of 8 per cent of molybdenum, however, would necessitate a considerable increase in the stated amounts of nickel in order to produce a wholly austenitlc structure, which would in turn increase the cost of the steels substantially. However, I have found that surprisingly high strength at elevated temperature, in fact superior to any other austenitic steel known to me, is attained at about the middle of the range given for molybdenum, which permits the use of nickel within economically reasonable amounts while still obtaining the desired structure.

The carbon content should not exceed about 0.15 per cent, and preferably should not be over about 0.1 per cent because the hardness of the steels with not over about 0.1 per cent of carbon is such that they are readily machinable whereas machining difliculties may increase with increase in the carbon content. On the other hand, an increase in the carbon content makes it possible to reduce the amount of nickel needed to keep the steels austenitlc so that where machining properties are not of major importance the cost of the steels may be reduced by increasing the carbon content and correspondingly lowering the nickel content.

For most purposes I prefer alloys containing about 16 per cent of chromium, 25 per cent of nickel, 6 per cent of molybdenum, and not over about 0.1 per cent of carbon.

Nickel, chromium and molybdenum are, together with carbon, the essential alloying elements, and the remainder of the steel may consist of iron together with impurities and elements in amounts which do not essentially alter the characteristics of steels of the stated composition. Thus, manganese may be present within the amounts normal to such steels. It is of assistance, however, in producing an austenitic structure, it contributes to the forgeability of the steels, and it does not detrimentally affect the high temperature properties presently to be described in detail. Preferably, therefore, the

steels will contain from about 1 to about 2 per' cent of manganese. Silicon also may be Present in normal amounts. To increase the oxidation resistance it may be used in greater amounts but most suitably not over about 2 per cent because it tends to cause the production of ferrite so that when excess amounts of silicon are present the amount of nickel should be increased to maintain the structure austenitlc.

The alloys may, and with advantage will, contain nitrogen also. This element tends to produce a fine primary grain structure, which is beneficial to the forging properties; it is productive of precipitation hardening properties, which improves the high temperature strength; and, perhaps the most important of all, it tends to stabilize the austenite. The amount ofv nitrogen present in the alloys is largely a function of the chromium, which fixes it as a nitride. Consequently, the greater the amount of chromium the larger may be the amount of nitrogen present. Although exact limits can not be fixed which are applicable to all purposes, I now believe that for all practical purposes the steels may contain from about 0.05 to about 0.25 per cent of nitrosea. This element is introduced most simply in the production of these alloys by the use of high nitrogen ferrochrome.

Experience has shown that these alloys are exceedingly still, or hard, under the forging hammer, and that at temperatures above about 2200 F. they tend to .break up under the hammer. However, by normalizing them at about 2150 F. and then working lightly under the hammer at about 1900 F. followed by reheating to a somewhat higher temperature and again working lightly, and repeating this procedure until the dendritic structure of the ingot has been destroyed they may be forged satisfactorily at about 2100 or 2150 F.

In the as-forged condition these alloys contain an appreciable amount of a complex carbide phase which is distributed widely throughout the entire structure. This begins to go into solution at about 2000 F., and by heat treatment at about 2l50 to 2200 F. a fully austenitic structure is developed. To prepare the wrought articles for use they are therefore heated within that range and quenched, suitably into water. At about 2200 F. another phase appears in the grain boundaries which may explain the diiiiculties that are encountered in forging the alloys at such temperatures.

These alloys possess gOOd mechanical properties at room temperature, which is of course essential for fabrication and in use for stressed parts in- Ultimate strength 115,000 lbs/sq. in. Yield point 60,000 lbs/sq. in. Elnn afirm 50% in 2 in. Reduction of area 66% Hardness 179 Brinell Impact (Izod) ft. lbs.

It will be seen that the alloy thus combines excellent strength and high yield point with outstanding ductility and shock resistance. Furthermore, the hardness in the as-quenched condition is such as to insure good machining properties. In this condition the alloy is fully austenitic.

The high temperature properties of the alloys may be understood best by reference to data obtained by the actual testing of bars made from the foregoing heat and from another heat of quite similar composition. Their creep strength was determined at 1300 and 1400" F. using stresses of 10,000 and 15,000 pounds per square inch at both temperatures. These tests were conducted for at least 1000 hours in accordance with the standard procedure of the A. S. T. M. The creep asearos rates obtained from the time elongation curves were as foliows:

Stress, Creep rate Temp" p s. %,1000 hrs.

when these data are plotted to logarithmic coordinates in comparison with the creep data for the 18-8 and 16-13-3 steels it appears that based on creep rates alone the steels provided by this invention are markedly superior to the best of the previously known austenitic steels, as may be observed from the following comparative tabula- The alloys of this invention are thus seen to be decidedly superior in creep strength to the best of the previously known high temperature steels because they have about twice the load carrying ability at 1300 and 1400 F. of the16-13-3 steels, which are in turn substantially stronger than the well known 18-8 steels.

The superiority of these alloys for high temperature uses was determined also by' rupture tests carried out according to standard practice. At 1300 F. and undera load of 32,000 pounds per square inch-the bars ruptured after 170.75 hours, exhibitng 17.5 per cent elongation in 2 inches and 33.2per cent reduction of area. At

the same temperature under a load of 25,000 pounds per square inch rupture occurred after 1332 hours, the elongation being 3.5 per cent and the reduction of area 7.9 per cent. At 1400 F. the bars failed after 72 hours under a load of 24,000 pounds per square inch, with an' elongation of per cent'and a reduction of area of 20.1 per cent, while under a load of 16,000 pounds per square inch rupture occurred after 2916 hours, the elongation being 2 per cent and the reduction of area 1.5 per cent. By plotting these data to logarithmic coordinates the rupture strengths for various times can be determined. These are indicated in comparison with the best of the previously available austenitic steals in the following tabulation: 1

with a load of 32,000 pounds per square inch at 1300 F. the steel of this invention would fail in about 171 hours, 16-13-3 would fail in about 7 hours, and 188 would fail in a few minutes. Similarly with a load of 16,000 pounds per square inch at 1400 F. the steel of this invention would fail after 2,916 hours, while 16-13-3 would fail in about 100 hours and 18-8 in about 3.5 hours.

These results show, without further discussion.

that the alloys provided by this invention are markedly superior in rupture strength to the best of the previously available austenitic steels used for high temperature work.

Moreover, based upon tests of the creep test specimens after completion of the tests the alloys of this invention appear to possess ductility adequate for most purposes. The room temperature mechanical properties of the creep specimens after test were as follows:

Treatment oi specimens Pmpmy 1,300 F.- 1,400 F.

15.000# 15,000# 1,108 hrs. 1,152 hrs.

I Ultimate str., p. s l 133,000 125,500 Yield, 0.1 p. s 1 69,500, 62,250 Yield, 0. p. s 73, 500 65,500 Prop. lim t, p. s 47, 500 45,000 Elong, percent 2 20. 0 l7. 5 Red. area, percent.. 25. 4 18. 4

h It will be observed that the strength after creep testing was greater than that of the original specimens. This is due, apparently, to precipitation hardening which occurs during prolonged exposure at elevated temperatures, and the microstructures of the specimens after creep testing showed a very fine general precipitation of carbides and perhaps of nitrides also where the steels contain nitrogen as described. As will be observed, however, hardening had not occurred under the test conditions to point such as to render the alloys deficientin ductility for high temperature purposes.-

According to the provisions of the patent statutes, I have explained the principle and method of practicing my invention and have described what I now consider to represent its best embodiments. However, I desire to have it understood that, withinthe scope of the appended claims, the invention may be practiced otherwise than' as specifically described. I claim:

1. An iron alloy comprising about 12 to 20 per cent of chromium, about 4 to 8 per cent of molybdenum, not over about 0.15 per cent of carbon, and nickel in an amount such as to render the alloy austenitic, and the remainder iron together with impurities and elements in amounts the alloy stated, and characterized by being austenitic and free from delta iron when quenched Temp ".,i. from about 2150 F., and by high creep and rup- F. ture strengths at a temperature of 1300 F.

10 1,000 10,000 100,000 55 2. An iron alloy comprising about 16- per cent of chromium, about 25 per cent of nickel, about 6 This invention 1, 300 34,000 20,000 20,000 15,500 per cent of molybdenum, up to about 2 per cent 13 11 1111111 113% 131% i iggg 3% 21% g g of manganese, about 0.05 to 0.25 per cent of ni- I trogen, not over about 0.15 per cent of carbon, and f ii ggffffff {13g 35113 2% 11% o the remainder iron together withimpurities and l8-8 1,400 13,800 9,500 0,400 4,000 2,000 elements in amounts which do not essentially The benefits .of this invention may be understood by reporting the data just given in another manner. For instance, these data mean change the character of the alloy stated, and characterized by being austenitic and tree from delta iron when quenched from about 2150 E,

that u by high creep and rupture strengths at a temperature of 1300 F2, and by high resistance to oxidation at elevated temperatures.

3. As a new article of manufacture, a wrought article for use at high temperatures under'stress, said article being formed from an iron alloy comprising about 16 per cent of chromium, about 25 per cent of nickel, about 6 per cent of molybdenum, up to about 2 per cent of manganese, not over about 0.15 per cent of carbon, and the remainder iron together with impurities and elements in amounts which do not essentially change the character of the alloy stated, and characterized by being austenitic and free from delta iron when quenched from about 2150 F., by high creep and rupture strengths at a temperature of 1300" F., and by high resistance to oxidation at elevated temperatures.

4. An iron alloy comprising about 15 to 20 per cent of chromium, about 14 to 35 per cent of nickel, about 4 to 8 per cent of molybdenum, up to about 2 per cent of manganese, not over about 0.15 per cent of carbon, about 0.05 to 0.25

per cent of nitrogen, and the remainder iron together with impurities and elements in amounts which do not essentially change the character of the alloy stated, the amounts of chromium,

' molybdenum, and nickel being related to each other to render the alloy austenitic, and characterized by being austenitic and free from delta iron and by high creep and rupture strengths at a temperature of 1300 F.

5.A steel comprising about 12 to per cent of chromium, about 4 to 8 per cent of molybdenum, about. 14 to 35 per cent of nickel, manganese in an amount between that normal to steels and 2 per cent, about 0.05 to 0.25 per cent of nitrogen, carbon, and the remainder iron together with impurities and elements in amounts which do not essentially change the character of the steel, and characterized by being austenitic and free from delta iron when quenched from about 2150 F. and by then having in solution in th austenite carbides in a maximum amountv corresponding to about 0.15 per cent of carbon, and characterized further by high creep and rupture strengths at a temperature of 1300 F.

6. A steel comprising about 16 per cent of chromium, about 25 per cent of nickel, about 6 per cent of molybdenum, together with carbon, and the remainder iron together with impurities and elements in amounts which do not essentially change the character of the steel, and characterized by being austenitic and free from delta iron when quenched from about 2150 F. and by then having in solution in the austenite carbides in a maximum amount corresponding to about 0.15 per cent of carbon, and characterized further by high creep and rupture strengths at a temperature of 1300 F.

7. A steel comprising about 16 per cent of chromium, about 6 per cent of molybdenum, about 25 per cent of nickel, together with carbon, and the remainder iron together with impurities and elements in amounts which do not essentially change the character of the steel, and characterized by being austenitic and free from delta iron when quenched from about 2150 F. and by then having in solution in the austenite carbides in a maximum amount corresponding to about 0.15 per cent of carbon, and characterized further by high creep and rupture strengths at a temperature of 1300" F. x

8. A wrought steel article formed from steel containing about 12 to 20 per cent of chromium, about 4 to 8 per cent of molybdenum, about 14 to per cent of nickel, about 0.05 to 0.25 per cent of nitrogen, manganese in an amount not exceeding about 2 per cent, together with carbon, and the remainder iron together with im purities and elements in amounts which do notessentially change the character of the steel, and characterizedby' beingaustenitic and free from delta iron when quenched from about 2150 F. and by then having in solution in the austenite carbides in a maximum amount corresponding 40 to about 0.15 per cent of carbon, and characterized further by high creep and rupture strengths at a temperature of 1300 F.

MARTIN FLEISCHMANN. 

